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Sound waves, safely treats cancer

The findings were presented at the 168th meeting of the Acoustical Society of America (ASA) in Indianapolis in October.The technique in question, known as histotripsy, uses ultrasound to mechanically destroy cancerous or other targeted tissues. This is a departure from traditional ultrasound therapy, which destroys tissues using heat.

Tumor-destroying technique uses currently available technology

In histotripsy, ultrasound-induced vibration leads to the production of bubbles formed from dissolved gases. If the vibration continues at high enough intensities, the bubbles eventually collapse, release a shock wave that can completely liquefy cells. A series of these collapses (known as inertial cavitation) can destroy a vast section of tissue, such as a tumor. Scientists can accelerate the process by injecting microbubbles into the tissue before the procedure.

Studies have shown that histotripsy can totally liquefy tumors, and can do so with remarkable precision and minimal impact on healthy surrounding tissue.

To date, three forms of histotripsy have been developed. The original method, shock scattering histotripsy, uses high intensity pulses ranging in length from 2 to 20 microseconds. More recently, researchers have developed an even more high intensity form, intrinsic threshold histotripsy, which uses pulses only 0.1 to 2 microseconds long.

The University of Washington researchers were experimenting with the third form, known as boiling histotripsy, which combines vibration and heat to produce the same effect as more conventional histotripsy. The main advantage of this technique is that it requires much less energy, and can be more easily produced by adapting existing technology.

In using boiling histotripsy to eliminate cancerous tumors, the researchers expected that once the tumor was liquefied, the body would clear away all traces of it as cellular waste. To their surprise, however, they found that while the tumors in their experiments were indeed utterly destroyed, in some cases the underlying extracellular structure remained.

"In some of our experiments, we discovered that some of the stromal tissue and vasculature was being left behind," researcher Yak-Nam Wang said. "So we had the idea about using this to decellularize tissues for tissue engineering and regenerative medicine."

May hold the key to organ regeneration

The surviving structure, known as the extracellular matrix, is the fibrous network that cells use to guide their growth. An intact extracellular matrix could hypothetically be seeded with stem cells, allowing the regrowth of new, healthy tissue.

For that reason, scientists have searched for decades for a way to remove tissue without destroying the extracellular matrix. Yet nearly all methods currently used to destroy cells also damage the surrounding tissues and fibers. They also take much longer than histotripsy.

"In tissue engineering, one of the holy grails is to develop biomimetic structures so that you can replace tissues with native tissue," Wang said.

Wang suggested that the new technique could even be used to create extracellular matrices to implant in other parts of the body that suffered damage long ago.

"The other thought is that maybe you could just implant the extracellular matrix and then the body itself would self-seed the tissues, if it's just a small patch of tissue that you're replacing," Wang said. "You won't have any immune issues, and because you have this biomimetic scaffold that's closer to the native tissue, healing would be better, and the body would recognize it as normal tissue."

Wang is now researching decellularization of kidney and liver tissue in large animals. He then hopes to move on to assessing the regenerative ability of large decellularized tissue.

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